Typically, the effluent slurry (after digestion) from an anaerobic digester is separated into two portions, including a solids portion containing most of the phosphorus and a liquid portion containing most of the nitrogen and potassium. The liquid portion is normally stored in a holding pond for land application or returned by pipeline to an aerobic treatment facility. The solid portion is normally removed, collected, and used for animal bedding or disposed in a landfill, sold for horticulture use, or used in a land application.
The consequences of the typical anaerobic process is that neither separated solid nor liquid effluent portions, once separated, can be used as a “balanced” fertilizer since a majority of the nitrogen and potassium resides with the liquid and a majority of the phosphorus resides with the solids. A balanced fertilizer is one that contains nutrients that a plant needs to grow, such as potassium, sulfur, phosphorus, and nitrogen, in a ratio similar to the influent slurry.
A conventional arrangement for anaerobic digestion of concentrated waste substrates, such as sludge, food waste, or animal manure, is illustrated in FIG. 1. The concentrated waste slurry 102 is normally diluted with water 106 if the influent solids concentration is greater than the desirable maximum digester solids concentration, which is normally less than 12%. The slurry 102 and water 106 are mixed in the mixing area 104 to produce mixed slurry, which is then digested in any of a variety of anaerobic digesters 108. The effluent slurry 110 from the digester 108 is then separated by a separation device 112 into a solid portion 114 and a liquid portion 116 for disposal.
As FIG. 1 shows, high solids anaerobic systems produce two effluent streams including a solid and liquid stream, neither of which contains nutrients in the ratio similar to the influent stream. Other prior art systems are variations of FIG. 1. For example, single solid effluent systems and single liquid effluent systems are possible if separation 112 is not carried out.
A single solid effluent stream may be created by minimizing or limiting the amount of anaerobic digestion that occurs so that the effluent solids concentration is substantially the same as the influent solids concentration. However, this option is inefficient because there is relatively little conversion of the solids. If, on the other hand, a reasonable digestion detention time is allowed, anaerobic digestion can convert more than 20% of the solids to gas, resulting in a liquid effluent stream. However, liquid streams are generally not desirable because of the difficulty of handling and the disposal of liquid streams.
Because the cost of transporting liquid effluent greatly exceeds the cost of transporting a solid dewatered product, sometimes a solid slurry stream, even with low conversion of solids in the digester, will be produced. Conditions that may require a solid slurry stream include (1) land may not be available or in close proximity for the disposal of the separated liquid portion, (2) the cost of hauling the liquid portion to some disposal site is excessive, (3) the discharge of the liquid portion containing high nutrient concentrations is limited by the wastewater treatment facility's ability to treat the liquid or the receiving ground or surface waters cannot receive the effluent nutrient load, and (4) there is a need to produce a highly valued balanced fertilizer containing the nitrogen, phosphorus, potassium, and sulfur for crop growth and pollution abatement.
However, the prior art systems have failed to provide a dewatered solids product with high solids conversion in the anaerobic digester and wherein the solids product can be used as a balanced fertilizer. If a balanced fertilizer is not available for agricultural crops, the nutrients from an unbalanced fertilizer when applied in excess of the limiting nutrient will eventually pollute the environment through runoff, volatilization, or erosion. Consequently, a balanced fertilizer would be advantageous.